We propose to initiate a comprehensive investigation of muscle development in two taxa of basal meatzoans: the cnidarians and ctenophores. Our long-range goal is to determine if a defined cell type (muscle cells) evolved once in metazoan evolution, or whether contractility evolved independently in distinct evolutionary lineages. In this study, we will isolate, characterize, and determine the expression pattern of genes that 1) specify the muscle cell phenotype, and 2) specify the formation of definitive muscle cells in the sea anemone Nematostella vectensis (Cnidaria: Anthozoa) and the ctenophore Mnemiopsis leidyi. If contractility evolved independently in metazoans, it is unlikely that all of the members of the ontogenic pathway would be as conserved as structural genes might be due to functional constraints. We have three specific aims in this proposed study. First, we will isolate and characterize muscle differentiation genes in N. vectensis and M. leidyi using PCR. We will then use these sequences for phlyogenetic analyses and to construct muscle-specific markers in further developmental studies. Second, we will determine the normal expression patterns and function of these muscle-specific genes in cnidarian and ctenophore embryos and adults using in situ hybridization with specific riboprobes based on the gene sequences isolated in (1) and using immunohistochemistry with monoclonal antibodies to the derived amino acid sequences of the protein products. Third, we will compare the expression and function of muscle-specific genes during whole animal regeneration with normal muscle-specific gene expression and function again using in situ hybridization and immunohistochemistry. The results of this study will reveal details of the increasing complexity of the muscle cell developmental program and defects associated with deviations in the molecular regulation of muscle cell function.